Porous sheet materials are used in the filtration of water, wastewater, and other fluids. For example, such filtration materials are used to remove dirt, dust, particulates, suspended solids, heavy metals and other matter from liquid streams. Porous sheet materials are also used in applications where it is necessary to filter out microbes such as spores and bacteria. For example, porous sheet materials are used in the packaging of sterile medical items, such as surgical instruments. In sterile packaging, the porous packaging material must be porous to gases such as ethylene oxide that are used to kill bacteria on items being sterilized, but the packaging materials must be impervious to bacteria that might contaminate sterilized items. Another application for porous sheet materials with good barrier properties is for making pouches that hold moisture absorbing desiccant substances. Such desiccant pouches are frequently used in packaged materials to absorb unwanted moisture.
The physical properties of a fabric or sheet material determine the filtration applications for which the material is suited. It has been found desirable for sheet materials used in a variety of filtration applications to provide good barrier to the passage of fine particles but also have good permeability to gases and/or liquids. Another set of desirable properties for fabrics or sheet materials used in certain filtration applications is that the material have enough strength and tear resistance that filters made using the sheet material will not lose their integrity under anticipated working conditions. Finally, most filter materials must have a manufacturing cost that is low enough to make the use of the material practical in low cost filters.
A number of standardized tests have been devised to characterize materials used in filtration and in sterile packaging so as to allow others to compare properties and make decisions as to which materials are best suited to meet the various anticipated conditions or circumstances under which a material will be required to serve. The strength and durability of sheet materials has been quantified in terms of tensile strength, tear strength and elongation. The primary tests used for characterizing filtration efficacy are tests that measure filter efficiency (% of particulates retained by a filter); resistance to water flow through a filter at a given flow rate (also known as clean permeability); and life of a filter material under a given loading and operation condition (also known as capacity). Barrier properties can be measured by both bacterial or particulate barrier tests.
TYVEK.RTM. spunbonded olefin has been in use for a number of years as a material for filtration and sterile packaging applications. E. I. du Pont de Nemours and Company (DuPont) makes and sells TYVEK.RTM. spunbonded olefin nonwoven fabric. TYVEK.RTM. is a registered trademark owned by DuPont. TYVEK.RTM. nonwoven fabric has been a good choice for filtration and sterile packaging applications because of its excellent strength properties, its good barrier properties, its reasonable permeability, its light weight, and its single layer structure that gives rise to a low manufacturing cost relative to most competitive materials. While TYVEK.RTM. spunbonded olefin has proved to have excellent barrier properties for filtration of water and wastewater, its limited permeability requires differential pressures across the filter media that are larger than is desirable. Similarly, while TYVEK.RTM. spunbonded olefin has proved to have excellent barrier properties for sterile packaging, the material's relatively low permeability lengthens the cycle times needed for injecting and removing sterilizing gases during sterilization procedures.
Thus, there is a need for a sheet material suitable for use in filtration and sterile packaging that has strength, weight and barrier properties at least equivalent to that of the TYVEK.RTM. spunbonded olefin nonwoven sheet material that has been traditionally used for such applications, but that also has significantly improved air and liquid permeability to make use of the material as a filtration material more efficient.